1. Field of the Invention
The present invention relates generally to an alarm system having a capability of testing, on a regular basis, the resistance of the circuits of alarm condition resistor elements in the alarm system to detect when a circuit has degraded to a pre-trouble resistance range, such that the circuit of the resistor element can be serviced and repaired prior to its degrading to a point which will trigger the alarm system. For instance, a typical zone alarm circuit having a normal resistor element might have a resistance of from 150-450 ohms, and trigger an alarm, normally by the resistor element opening, when the resistance of the zone alarm circuit exceeds 550 ohms. This allows a pre-trouble resistance range of 450-550 ohms, which signals the necessity for service on the zone alarm circuit.
2. Discussion of the Prior Art
The prior art is replete with many diverse and different arrangements for testing the integrity of alarm systems, some of which are similar in some respects, as noted by the comments hereinbelow, to particular aspects of the present invention. For instance, Lofgren U.S. Pat. No. 3,882,476 discloses an alarm system which performs periodic tests of all alarm trigger amplifiers. Dow et al U.S. Pat No. 3,886,413 discloses an alarm system which performs a time division multiplexing of the testing mode cycle, with particular time periods corresponding to particular tests of the system. Yoshizaki U.S. Pat. No. 4,489,312 discloses a fire alarm system which tests remote fire detectors, each having a separate address code. Responsive to the receipt of the proper address, a test circuit at the detector applies a test voltage to the detector, which tests and resets the detectors. Sasaki U.S. Pat. No. 4,506,255 performs a testing sequence in which different voltage levels are applied during the testing sequence.
Tanaka et al U.S. Pat. No. 4,518,952 is also considered to be fairly pertinent to the present invention, and provides a test circuit for an alarm system having a transmission line, a plurality of terminal units connected to one end of the transmission line in a distributed manner, and a receiver connected to the other end of the transmission line. Each of the plurality of terminal units includes a sensor, an amplifier, an A/D converter, an interface, a test voltage generator, and a control circuit. A test circuit is connected between positive and negative power source terminals, and an analog output is formed of a test voltage and a normally detected voltage. This output is generated by the sensor, and is converted into a digital output by the A/D converter which is applied to the receiver when the receiver supplies its address signal and a test instruction signal to a corresponding one of the terminal units. However, this reference is also different from the present invention in several significant areas. It performs the resistance measurement at the module, rather than at the central panel, as in the present invention, the system is not interconnected by only two wires similar to the subject invention, and the modules are not supplied with all of their electrical power over those same two wires.
In general, none of the prior art discussed hereinabove discloses a concept similar to the present invention of a time division multiplexing arrangement of an addressed module wherein a reference resistor is connected across a wire line pair in one time period, and a resistor alarm circuit is connected across the wire line pair in another time period, to enable the actual resistance of the resistor alarm circuit to be measured while monitoring for a pre-trouble resistance range. Additionally, none of the prior art discussed hereinabove discloses a concept of dropping the voltage levels across the wire line pair during the actual resistance tests, for the purpose of isolating the module from the wire line pair during the tests.